title: Analyzing the catalytic mechanism of the Fe-type nitrile hydratase from Comamonas testosteroni Ni1.
abstract: In order to gain insight into the catalytic mechanism of Fe-type nitrile hydratases (NHase), the pH and temperature dependence of the kinetic parameters k cat, K m, and k cat/ K m along with the solvent isotope effect were examined for the Fe-type NHase from Comamonas testosteroni Ni1 ( CtNHase). CtNHase was found to exhibit a bell-shaped curve for plots of relative activity vs pH over pH values 4-10 for the hydration of acrylonitrile and was found to display maximal activity at pH approximately 7.2. Fits of these data provided a p K ES1 value of 6.1 +/- 0.1, a p K ES2 value of 9.1 +/- 0.2 ( k' cat = 10.1 +/- 0.3 s (-1)), a p K E1 value of 6.2 +/- 0.1, and a p K E2 value of 9.2 +/- 0.1 ( k' cat/ K' m of 2.0 +/- 0.2 s (-1) mM (-1)). Proton inventory studies indicate that two protons are transferred in the rate-limiting step of the reaction at pH 7.2. Since CtNHase is stable to 25 degrees C, an Arrhenius plot was constructed by plotting ln( k cat) vs 1/ T, providing an E a of 33.3 +/- 1.5 kJ/mol. Delta H degrees of ionization values were also determined, thus helping to identify the ionizing groups exhibiting the p K ES1 and p K ES2 values. Based on Delta H degrees ion data, p K ES1 is assigned to betaTyr68 while p K ES2 is assigned to betaArg52, betaArg157, or alphaSer116 (NHases are alpha 2beta 2 heterotetramers). Given the strong similarities in the kinetic data obtained for both Co- and Fe-type NHase enzymes, both types of NHase enzymes likely hydrate nitriles in a similar fashion.
authors.affiliation: Department of Chemistry, Loyola University--Chicago, 1068 West Sheridan Road, Chicago, Illinois 60626, USA.
authors.forename: Saroja
authors.initials: S
authors.lastname: Rao
authors.forename: Richard C
authors.initials: RC
authors.lastname: Holz
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D002384:Catalysis; D020604:Comamonas testosteroni / Q000201:enzymology*; D006836:Hydro-Lyases / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry; D007700:Kinetics; D009570:Nitriles / Q000737:chemistry; D011522:Protons
chemical_list: D001426:Bacterial Proteins; D009570:Nitriles; D011522:Protons; D007501:Iron; D006836:Hydro-Lyases; C048831:nitrile hydratase